Shoes of all types are subject to great amounts of stress through repeated, cyclical loading caused by walking, running, and other activities. Athletic shoes generally experience even more acute stress due to the higher impact levels associated with athletic activities. The repeated impact of a high-intensity gait causes great wear and even break-down of many shoes. In addition, the impact is stressful to the wearer's body. Without proper support and cushioning, the foot, ankle, calf, knee, and even hip joints are challenged physically by athletic activity. Proper alignment of joints, bones, and muscles of the foot, leg, and hip is crucial. A shoe that is improperly constructed, worn down, or improperly calibrated to the activity can cause off-axis loading of joints and bones. Off-axis loading can cause fatigue and tension to the wearer.
There have been many attempts to create a shoe sole that provides a durable, long lasting, and reliable support to the wearer throughout even the most vigorous athletic activity. Many conventional materials generally sacrifice responsiveness for comfort, or comfort for responsiveness. Also, most materials are best suited either for stressful, high-impact activity such as running, or toward lower-level activities such as standing or walking. Many of previous attempts place a bladder or insert in the sole containing air, gel, plastic, or other material to absorb energy from impact. These materials generally cannot provide a range of response characteristics to different levels of pressure and impact. In other words, a softer soled shoe that may be well suited for standing and walking is not properly calibrated for higher-impact levels. Similarly, a stiffer shoe that may provide proper resiliency and performance for running or other high-energy activities is generally not well suited—even uncomfortable or painful—for lower level activities.
Some attempts have been made to provide a shoe with dilatant (i.e., shear-thickening) materials in the sole. These materials increase in viscosity as a function of the rate of shear (e.g., silly putty). Dilatant materials, however, are generally not accurately calibrated to the responsiveness required for multiple levels of activity to provide optimal responsiveness and comfort. Also, dilatant materials cannot be readily injection molded or compression molded, increasing the complexity and cost of manufacture. There is a need for a shoe assembly that can meet the needs of both high- and low-intensity activities without sacrificing comfort or performance.